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package javax.el;

import java.io.Serializable;

/**
 * Base class for the expression subclasses {@link ValueExpression} and
 * {@link MethodExpression}, implementing characterstics common to both.
 *
 * <p>All expressions must implement the <code>equals()</code> and
 * <code>hashCode()</code> methods so that two expressions can be compared
 * for equality. They are redefined abstract in this class to force their
 * implementation in subclasses.</p>
 *
 * <p>All expressions must also be <code>Serializable</code> so that they
 * can be saved and restored.</p>
 *
 * <p><code>Expression</code>s are also designed to be immutable so
 * that only one instance needs to be created for any given expression
 * String / {@link FunctionMapper}. This allows a container to pre-create
 * expressions and not have to re-parse them each time they are evaluated.</p>
 *
 * @since JSP 2.1
 */
public abstract class Expression
        implements Serializable {
    // Debugging
    
    /**
     * Returns the original String used to create this <code>Expression</code>,
     * unmodified.
     *
     * <p>This is used for debugging purposes but also for the purposes
     * of comparison (e.g. to ensure the expression in a configuration
     * file has not changed).</p>
     *
     * <p>This method does not provide sufficient information to
     * re-create an expression. Two different expressions can have exactly
     * the same expression string but different function mappings.
     * Serialization should be used to save and restore the state of an
     * <code>Expression</code>.</p>
     *
     * @return The original expression String.
     */
    public abstract String getExpressionString();
    
    // Comparison
    
    /**
     * Determines whether the specified object is equal to this
     * <code>Expression</code>.
     *
     * <p>The result is <code>true</code> if and only if the argument is
     * not <code>null</code>, is an <code>Expression</code> object that
     * is the of the same type (<code>ValueExpression</code> or
     * <code>MethodExpression</code>), and has an identical parsed
     * representation.</p>
     *
     * <p>Note that two expressions can be equal if their expression
     * Strings are different. For example, <code>${fn1:foo()}</code>
     * and <code>${fn2:foo()}</code> are equal if their corresponding
     * <code>FunctionMapper</code>s mapped <code>fn1:foo</code> and
     * <code>fn2:foo</code> to the same method.</p>
     *
     * @param obj the <code>Object</code> to test for equality.
     * @return <code>true</code> if <code>obj</code> equals this
     *     <code>Expression</code>; <code>false</code> otherwise.
     * @see java.util.Hashtable
     * @see java.lang.Object#equals(java.lang.Object)
     */
    public abstract boolean equals(Object obj);
    
    /**
     * Returns the hash code for this <code>Expression</code>.
     *
     * <p>See the note in the {@link #equals} method on how two expressions
     * can be equal if their expression Strings are different. Recall that
     * if two objects are equal according to the <code>equals(Object)</code>
     * method, then calling the <code>hashCode</code> method on each of the
     * two objects must produce the same integer result. Implementations must
     * take special note and implement <code>hashCode</code> correctly.</p>
     *
     * @return The hash code for this <code>Expression</code>.
     * @see #equals
     * @see java.util.Hashtable
     * @see java.lang.Object#hashCode()
     */
    public abstract int hashCode();
    
    /**
     * Returns whether this expression was created from only literal text.
     *
     * <p>This method must return <code>true</code> if and only if the
     * expression string this expression was created from contained no
     * unescaped EL delimeters (<code>${...}</code> or
     * <code>#{...}</code>).</p>
     *
     * @return <code>true</code> if this expression was created from only
     *     literal text; <code>false</code> otherwise.
     */
    public abstract boolean isLiteralText();
}